In recent years, as a display, a liquid crystal display apparatus using a surface light source device has been remarkably widespread. As the liquid crystal display apparatus using the surface light source device, for example, a liquid crystal display apparatus including an edge light-type surface light source device is known. In such a liquid crystal display apparatus, light emitted from a light source enters a light guide plate, and propagates through an inside of the light guide plate while repeating a total reflection on a light output surface (liquid crystal cell-side surface) of the light guide plate and a back surface thereof. A part of the light that propagates through the inside of the light guide plate allows a traveling direction thereof to be changed by a light scattering body or the like, which is provided on the back surface of the light guide plate or the like, and is output from the light output surface to an outside of the light guide plate. Such light output from the light output surface of the light guide plate is diffused and condensed by various optical sheets such as a diffusion sheet, a prism sheet, a brightness enhancement film, or the like, and thereafter, the light enters a liquid crystal panel in which polarizing plates are arranged on both sides of a liquid crystal cell. Liquid crystal molecules of a liquid crystal layer of the liquid crystal cell are driven for each of pixels to control transmission and absorption of the incident light. As a result, an image is displayed.
As described above, the liquid crystal panel includes the polarizing plates on both sides (front and back) thereof, accordingly, approximately a half of the light that enters the liquid crystal panel is absorbed by the polarizing plate on the incident side, and utilization efficiency of the light in the liquid crystal panel is essentially low. Accordingly, when a larger amount of light is attempted to be entered into the polarizing plate in order to obtain desired brightness, there are such various problems that heat from the light source adversely affects the liquid crystal and the like, resulting in difficulty viewing display, in addition to a problem in that a power consumption of the light source is increased.
Various proposals have been made in order to improve such light utilization efficiency in the liquid crystal display apparatus. As one of the proposals, there is a proposal, in which non-polarized light from the light source is separated by using a polarized light separating body that separates the non-polarized light into two pieces of linearly polarized light, which are in a perpendicular relationship to each other, by transmission and reflection, one piece of the polarized light thus separated is transmitted and directly used, and in addition, the other piece of the reflected polarized light is also reused. That is, this is a technology in which, among polarized light components separated by the polarized light separating body, one of the polarized light components, which is transmitted, allows a polarization direction of the transmission light and a transmission axis direction of a lower polarizing plate (incident-side polarizing plate) to coincide with each other, and is caused to enter the liquid crystal cell, and other polarized light component is returned to the light source side, linearly polarized state of the returned light is eliminated therefrom by birefringence, reflection, diffraction or diffusion, or the like, and the light is guided to the polarized light separating body again and is reused so that the light utilization efficiency is enhanced. For example, in Patent Literature 1, a backlight is described, in which such a light control sheet that allows output light to be substantially vertical to a surface of the planar light guide plate is provided on a light output surface side of the planar light guide plate, and polarized light separating means is further arranged thereon.
However, in the backlight described in Patent Literature 1, a structure of the polarized light separating body is complicated, and in particular, it is difficult to form a polarized light separating layer on an inclined surface portion of a columnar prism array that is triangular in cross section, with the result that mass productivity of the backlight is insufficient. In recent years, there has also been developed a surface light source device constructed so that the light output from the light guide plate has a predetermined polarization state or the like. However, output light that is the polarized light output from the light guide plate is not utilized sufficiently, effectively, and sufficient brightness is not obtained.
Moreover, in Patent Literature 2, it is described that light polarized in a predetermined direction is output from a surface light source, and a base having a birefringence ratio, such as a biaxially stretched film, is used as a base of a prism sheet, and thus the polarization direction of the polarized light that enters the liquid crystal panel is controlled to reduce the light absorbed to the polarizing plate and enhance the utilization efficiency of the light. However, also in the liquid crystal display apparatus of Patent Literature 2, the utilization efficiency of the light is still insufficient.